Nat Gateway-based Embedded Operating System Vxworks Development

NAT is a kind of standard technology used to map one IP address realm into another. NAT enable a computer inside an institution to connect to the Internet with less or less legal IP address. Different subnets can use same private IP addresses to access the Internet, so the number of public IP address is reduced. At the same time, NAT is not only used as a method to resolve the lack of legal IP addresses, but also in network security, load balancing, virtual routing and many other fields. It is used broadly in TCP/IP based applications.In this paper, I first introduce some basic conceptions and working principle about NAT, analyze different types of NAT, depict the Gateway technology, and sum up the solution and work modes of NAT.Based on the previous jobs, I focus on the design of NAT system, by analyzing the characteristic of MPC860 and the network property of VxWorks, I present a design of NAT system based on the VxWorks. The design has proved compatible, extensible and transplantAccording to the research, the development of NAT system is based on VxWorks. This system has a function of dynamic address conversion and controlling time for the NAT map. Hooks functions provided by VxWorks are used to capture packets in the network, and deal with the packets. Meantime, the system realizes the control of NAT map table. Recording to the live time of NAT table items, system will delete the items rarely used, so, the problem of memory space usage is resolve. And semaphore is used to deal with the memory mutex.At last, some experiments are performed to test the system, the result of the test show that, by this system, computers in LAN could access to the internet with local address transparently, and the system works stable and reliable.